1. Field of the Invention
This invention is directed to a unique filtration system, in general, and to a filtration system for filtering “blue smoke” oil droplets from the atmosphere, in particular.
2. Prior Art
Blue Smoke is actually tiny oil droplets that make-up the blue haze that is typically associated with paving as well as the characteristic asphalt odor. Air pollution control agencies are becoming more concerned with blue smoke, especially as rubberized asphalt, recycled asphalt pavement (RAP) and polymer blends are more routinely specified. These specialty mixes are often known to produce an increased amount of “Blue Smoke.” More Blue Smoke usually causes a greater number of neighborhood complaints for visible emissions and odor, all of which results in more frequent inspections and a higher potential for being cited by air pollution agencies.
Typical systems include electrostatic precipitators, heating ventilating and air conditioning (HVAC) type filter units, baghouse-type devices with a bag coating substance which resembles “kitty litter,” and a recycle system that directs the blue smoke laden air into the combustion zone of the asphalt plant burner, with the intent to incinerate the oil droplets. Each of these technologies and the shortcomings thereof are briefly summarized in the following sections.
Electrostatic precipitators. This technology makes use of a series of cathodes and anodes, which are positioned to cause the oil droplets in the air to take on an electrical charge. The charged oil droplets are attracted to and then adhere to the opposite charged plates. As the oil accumulates on the plates, it builds up and then drips down into an oil collection chamber.
This technology has several shortcomings. In addition to being very expensive when compared to other technologies, it is somewhat dangerous when not properly maintained. Electrostatic precipitators are known to catch fire. They are also somewhat cumbersome to maintain. Special technicians are required to service the complex electronic circuitry. In addition, the clean up procedures include a periodic wash down process that creates a quantity of contaminated wash water that requires special disposal considerations.
Heating, ventilating and air conditioning filter units (HVAC). This technology makes use of a series of HVAC type filters that use a filter media to catch the oil droplets entrained in the air.
This technology has several shortcomings. Although the initial cost is considered moderate, its collection efficiency lowers with continued use, thus, rendering a higher operating cost if the unit is to operate at maximum efficiency. The problems stem from the typical horizontal airflow design of the unit. The horizontal airflow design causes the filters to load up with oil. As the filters load up, the air, instead of passing through the filters, goes around the filters. This phenomenon is generally referred to a “blow by,” and is a major problem for this technology. When the filters load up they become saturated and must be replaced to assure proper collection efficiency.
Baghouse device with a kitty litter bag coating component. This technology makes use of a baghouse-type device that is made up of a number of bags composed of a fabric filter media, similar in concept to a huge vacuum cleaner and are typically utilized for collecting dust from dust generating activities. As such, they are not suitable for capturing oil droplets entrained in an air stream. In this case, the aforementioned coating envelopes each fabric bag and absorbs the oil entrained within the air stream. When the coating becomes saturated with oil the baghouse collector is shut down and the coating is shaken off the bags or a jet of air is injected into each bag to blow off the saturated material. The baghouse device is designed with a collection hopper to hold the saturated coating material. When the hopper fills, the collector must be opened up and the coating material must be removed for proper disposal.
This technology has significant shortcomings. The technology is known to be moderate to high priced and to require a considerable amount of hands-on operational requirements. In order to assure effective collection efficiency, the equipment operator must monitor the pressure drop across the filter bags. When the reading indicates that the coating substance is saturated, the operator much schedule downtime when the material can be removed from the unit and replaced with clean materials. The saturated coating material must then be properly marked for environmentally appropriate disposal.
Recycle system that directs the blue smoke laden air into the combustion zone of the asphalt plant burner. This technology makes use of a booster fan that draws the oil-laden air from the top of the asphalt silos or the silo loading equipment. The oil-laden air is injected into the combustion zone of the asphalt plant burner. The burner is designed to deliver enough heat into the asphalt aggregate to drive off excess moisture and raise the aggregate to specification temperature. The burner is designed to transfer heat into the aggregate and it is not designed to function as an incinerator. The process functions reasonably well to remove the blue smoke from the top of the silo and silo loading equipment, but it is not an efficient incinerator.
This technology has significant shortcomings. The inefficient incinerator feature results in excessively high emissions of other air pollutants. In addition, the amount of exhaust air that can be sent to the combustion zone is limited. This inherent limitation means that although air at the top of the silo can be controlled, the air at the larger and more visible truck loadout area cannot be controlled.